1. Field of the Invention
This invention relates to a liquid crystal display apparatus for use in various information display terminal units and image display units. More particularly, it relates to a liquid crystal display apparatus for monochromatic display without coloration and for color display high in display contrast ratio (the ratio of quantity of transmitted light per unit area of the liquid crystal layer between when applied with voltage and when not applied).
2. Description of the Prior Art
The liquid crystal display apparatus is used in a wide variety of fields, including the watch, pocket calculator, computer terminal, word processor display, television, and others. More recently, there have been increased demands for multi-color and full-color displays of the liquid crystal display apparatus. They have already been realized in the fields of graphic display and image display. The color display widely realized at the present is that involving a method of installing color filter layers inside the liquid crystal cells, and developing colors by switching of the light by means of the liquid crystal cells. The mainstream of the display mode of this method is the twisted nematic (TN) display mode in which the axial direction of liquid crystal molecules is twisted by 90 degrees in consideration of the contrast, etc.
In the liquid crystal cells using the liquid crystals of the TN display mode, the dependency of the transmitted light of liquid crystal cell on the wavelength is large. Hence, uniform switching of the light over the entire visible light region is impossible. In other words, in a liquid crystal display apparatus for color display, for example, where color filters of red, green and blue color are formed in patterns on one of the substrates of the liquid crystal cell, since the intensity of transmitted light passing through each color filter is not uniform, a favorable color display cannot be made. In particular, in the liquid crystal cell of the so-called normally-closed type, in which the light is shielded when voltage is not applied to the liquid crystal layer, with planes of polarization of two polarizer being set parallel to each other, leakage of light and coloration of transmission light, when not applied with voltage, become obvious.
In the case of color liquid crystal display apparatus for switching the light by using the TN display mode having such color filter layers, the liquid crystal driving methods may be roughly classified into two systems. One is the so-called active matrix driving system in which nonlinear elements such as diodes, or switching elements such as thin-film transistors, are disposed on each picture element of liquid crystal cell. The other is the direct-multiplexed driving system for applying a voltage to each electrode in time division, and displaying and driving, by forming transparent electrodes in a pattern, so as to cross and oppose each other on two substrates across a liquid crystal layer.
In the former method, that is, the active matrix driving system, since nonlinear elements or switching elements such as diodes and thin film transistors must be formed in the liquid crystal cell, the production process of the liquid crystal cell is complicated, and the cost becomes higher.
In the latter method, that is, the direct-multiplexed driving system, since the steepness of the change of optical characteristic of liquid crystal in the vicinity of the threshold of the applied voltage determines the contrast ratio, this point is particularly important in the liquid crystal cell of the TN display mode.
To improve this threshold characteristic, for example, it is proposed to use a liquid crystal cell employing a supertwisted nematic (STN) liquid crystal in which the twisting angle of liquid crystal molecules is 180 to 270 degrees, by a rubbing method, and to use a liquid crystal cell employing a supertwisted birefringence effect (SBE) liquid crystal in which the twisting angle is 270 degrees, by SiO oblique deposition techniques. In liquid crystal cells using these STN liquid crystal and, SBE liquid crystal, the change of light transmissivity in the vicinity of threshold of applied voltage is steep, and a high contrast ratio can be obtained even when driving at a high time division, increasing the number of transparent electrodes. Still further, the contrast ratio characteristic with respect to the viewing angle is close to an axis symmetry, as compared with the TN display mode. In liquid crystal cells using these STN and SBE liquid crystal, however, since the birefringence effect of liquid crystal is utilized, the dependency of the light transmissivity on the wavelength is larger than that of the liquid crystal cell of TN display mode. Thus, the problems of light leakage and coloration become more serious when applied in the full color display.
Conventionally, as the method for improving the wavelength dependency of the display characteristic of liquid crystal, various attempts have been made. One example is that of the optical mode interference method for optimizing the optical conditions in mutual relations including the birefringence (difference between refractive index in the direction parallel to the axial direction of liquid crystal molecule and refractive index in the vertical direction), twisting angle of liquid crystal, thickness of liquid crystal layer, spiral pitch of liquid crystal layer, pretilting angle, and angle between the polarization plane of the polarizer and longitudinal axis of adjacent liquid crystal molecules in the STN liquid crystal and SBE liquid crystal. Another method is that of and the so-called guest-host method for preventing light leakage and coloration by absorbing the light transmitting through the liquid crystal layer, even when the liquid crystal is shielded. This is done by adding dichromatic pigments for absorbing light of a specific wavelength to the liquid crystal. In both methods, however, the display is dark and the contrast ratio is not sufficient.